Silicon Nitride (Si3N4) is widely used in the fabrication of integrated circuits; for example, the Silicon Nitride finds application as a final passivation film, a mechanical protective structure, an etching stop layer, a hard-mask, a diffusion barrier, an anti-reflective coating, a gate or spacer dielectric, and so on.
Several methods are known in the art for depositing a layer of Silicon Nitride on a wafer of semiconductor material. In the Low Pressure Chemical Vapor Deposition (LPCVD) technique, the Silicon Nitride is deposited in a furnace at low pressure (0.1-0.2 Torr) and high temperature (700-900° C.). However, the deposition temperature is not compatible with many fabrication processes of the integrated circuits.
A different method is based on the Plasma Enhanced CVD (PECVD) technique. In this case, the Silicon Nitride is deposited using a plasma reactor, wherein precursor components of the Silicon Nitride are injected. A plasma is then generated using a Radio-Frequency (RF) power source working at 50 kHz-15 MHz, while the plasma is kept at a pressure of 0.1-10 Torr; the resulting plasma has a (relatively) low electron density, typically in the range from 108 to 1010 n/cm3.
The PECVD Silicon Nitride features good electrical qualities. However, its morphological characteristics create several problems in some applications. Particularly, the PECVD process is isotropic; therefore, the PECVD Silicon Nitride has a low filling capability. Moreover, the layers deposited with the PECVD technique show bumps in the area on or near any corner and step structure.
A new method recently investigated for depositing Silicon Nitride is based on the High-Density Plasma CVD (HDP CVD) technique. This technique uses a reactor with one or two RF power sources that work at high frequency (for example, 1-5 MHz), and wherein the plasma is kept at very low pressure (for example, 0.5-50 mTorr). As a result, the plasma in the HDP CVD process has a high-density, typically in the range from 1011 to 1012 n/cm3.
The HDP CVD Silicon Nitride features good morphological qualities. In fact, the HDP CVD process is anisotropic; therefore, the HDP CVD Silicon Nitride has a high filling capability. Moreover, in the HDP CVD process a sputter-etching (caused by an RF biasing power source working at very high frequency, such as 13.56 MHz) is simultaneous with the deposition; in this way, any bumps in the area on or near corners and step structures are removed.
Examples of methods for depositing Silicon Nitride using the HDP CVD technique are disclosed in “Comparison between HDP CVD and PECVD Silicon Nitride for Advanced Interconnect Applications”, J.Yota et al., 0-7803-6327-2/00 2000 IEEE, pages 76-78 and in “A comparative study on inductively-coupled plasma high-density plasma, plasma-enhanced, and low pressure chemical vapor deposition silicon nitride films”, J.Yota et al., J.Vac.Sci. Technol. A 18(2) March/April 2000 0734-2101/2000/18(2)/372/5 2000 American Vacuum Society, pages 372-375. Both documents, which are incorporated by reference, propose HDP CVD processes that are specifically designed to obtain Silicon Nitride with a composition almost stoichiometric (i.e., about 43% of Silicon and about 57% of Nitrogen); in any case, the amount of Hydrogen in the Silicon Nitride is kept as low as possible.
A drawback of the HDP CVD Silicon Nitride is that it features very poor electrical qualities. Particularly, the HDP CVD Silicon Nitride has a reduced breakdown strength; this adversely affects the reliability of the active oxides in MOS transistors. Moreover, the HDP CVD Silicon Nitride significantly increases a threshold voltage of the transistors. This prevents the exploitation of the HDP CVD Silicon Nitride in several applications.